In order to examine the influence of penicillin on rabbits oculated with syphilis when penicillin was injected at the period when the complement combining reaction (WaR) turned positive and observed the course of the syphilitic local state and the disappearance of spirochaeta pallida from the local area, especially the transition of the serum reaction after the injection, and obtained the following results: 1) The term needed for the serum reaction to turn negative was reduced in all of the cases in which syphilis were oculated. 2) In the cases in which 50,000 U. of penicillin were injected, WaR turned negative in 28.6 days, Murata's reaction in 39.3 days and Kahn's reaction in 45.6 days on average. 3) In the cases in which 100,000 U. were injected WaR turned negativ in 38.5 days, Murata's reaction in 52.5 days and Kahn's reaction in 66.5 days on average. 4) In the cases in which 200,000 U. were injected, the influence of the injection to the serum reaction appeared rapidly and the serum reaction turned negative in a comparatively short time, i.e., WaR in 21.3 days, Murata's reaction in 34.6 days and Kahn's reaction in 39.3 days. 5) Concerning the local state (syphilitic orchitis), the inflammation was absorbed in 22.8 days in 50,000 U. injected cases, in 19 days in 100,000 U. injected cases and in 13.3 days in 200,000 U. injected cases. 6) As to the disappearance of spirochaeta pallida from the local area, it could not be detected in 2.3 days in the 200,000 U. injected cases, in 8.6 days in the 50,000 U. injected cases and in 7.7 days in the 100,000 U. injected cases. 7) In the demonstration of spirochaeta pallida, it disappeared comparatively rapidly in the left rather than the right testicle. 8) The recurrens of the serum reaction and the local state could not be found. 9) In short, a definite and regular influence of penicillin could not be found in the 50,000 U. or 100,000 U. injected cases, and moreover, no difference between the 2 groups was demonstrated. 10) In the cases in which 200,000 U. were injected, a definite and remarkable influence of penicillin could be recognized.